1. Technical Field
The present invention relates generally to a method for measuring vibration-related characteristics and more particularly to a method for measuring brake pad assembly damping and resonant frequency.
2. Discussion
A longstanding problem associated with the use of vehicle brakes is the generation of annoying noise often referred to as brake squeal. Various brake system components, such as brake rotors or drums (hereinafter collectively referred to as simply xe2x80x9cbrake rotorsxe2x80x9d) and brake pad or shoe assemblies (herein after referred to as simply xe2x80x9cbrake pad assembliesxe2x80x9d), are generally considered to be the source of a variety of such noises and associated vibrations.
In order to reduce brake noise, brake rotors and brake pad assemblies have been manufactured using materials and processes which tend to reduce the vibrations produced during braking. Such advancements have generally been directed to providing a consistent friction coefficient between the braking surfaces of the brake rotors and the brake pad assemblies, as well as controlling the frequency at which these components vibrate and their ability to damp vibrations. Typically, the greater the ability of a brake system component to damp vibrations, the less apt the part is to make undesirable noise during the operation of the vehicle.
Vehicle manufacturers and brake component manufacturers have attempted to control brake system noise by specifying a minimum amount or minimum level of vibration damping inherent in the brake system components. In the case of brake pad assemblies, damping is typically affected by the composition of the friction material (i.e., pad material) as well as the use of specific damping layers. The damping layers are typically formed from an insulating material that is secured to a brake pad backing plate via an adhesive bond to provide additional damping at critical (i.e., noise-related) frequencies. The damping performance of the insulating material is frequently dependent upon the quality of the adhesive bond.
Unfortunately, determination of the quality of brake pad and the damping layer usually involves a time consuming destructive test. Where the quality of the damping layer is analyzed, for example, the test typically requires that the insulating material be peeled off the brake pad backing plate, with the analysis being based on the amount of force exerted to remove the insulating material. While this testing appears to provide a satisfactory indication of the strength and durability of the adhesive bond between the insulating material and the brake pad backing plate, this testing only provides a crude indication of the insulating material""s damping performance.
Several non-destructive testing methods have been suggested wherein an accelerometer is secured to the brake pad assembly and the brake pad is subsequently excited by the impact of a hammer. The accuracy of the results obtained through these methods, however, is compromised since the accelerometer is mounted to the brake pad assembly during the test. Specifically, the accelerometer adds mass to the area of the brake pad assembly to which it is mounted, causing the brake pad assembly to respond in a manner that is not usually consistent with its performance in a brake system. Furthermore, the repeatability of the measurements is highly dependent upon the repeatability with which the accelerometer is positioned on the brake pad assembly. Small positional variances in the placement of the accelerometer tend to cause substantial variances in the measured damping of the brake pad assembly. Additional drawbacks associated with these tests are related to the efficiency with which the sample is excited by the impact hammer and the modal density of the sample.
Accordingly, there remains a need in the art for a method to accurately and repeatably test a brake pad assembly to measure its frequency response and damping capability. Ideally, the method should be xe2x80x9ccontactlessxe2x80x9d so that issues related to the mass-loading of the sample and the efficiency with which the sample is excited can be avoided. The method should also be quick and non-destructive. Such a method would provide an accurate measure of damping and frequency response, and would permit the identification of design and process variables that affect the performance of the brake pad assembly.
It is one object of the present invention to provide a method to accurately and repeatably test a brake pad assembly to measure its resonant frequency and damping capability.
It is a more specific object of the present invention to provide a nondestructive and contactless method to measure the resonant frequency and damping capability of a brake pad assembly.
In one preferred form, the present invention provides a method for measuring the vibration damping of a part. The method comprises the steps of vibrating the part with a contactless vibrator, measuring the output vibrations of the part with a contactless measuring device, determining an input frequency for testing the part, the input frequency being a frequency which uses the smallest gain to cause the part to produce vibrations having an output amplitude equal to a predetermined target amplitude, providing an input to the part with the contactless vibrator such that the part vibrates at the input frequency, simultaneously interrupting the input and using the contactless measuring device to measure as a function of time an output amplitude of vibrations in the part, and determining the rate at which the vibrations in the part are damped.